1. Field
Exemplary embodiments of the present invention relate generally to an image sensor, such as a complementary metal oxide semiconductor (CMOS) image sensor and, more particularly, to a CMOS image sensor and a method for driving a unit pixel of the image sensor.
2. Description of the Related Art
Typically, a unit pixel of a CMOS image sensor includes a photodiode, a transfer transistor, a reset transistor, a drive transistor, and a selection transistor. In the unit pixel, photo charges generated by the photodiode are transferred through the transfer transistor to a floating diffusion node, which is an input node (i.e., a gate terminal) of the drive transistor (i.e., a source follower transistor). That is a pixel signal i.e., an output voltage of the drive transistor, is driven by the voltage of the floating diffusion node.
As the size of the unit pixel is reduced, a pinning voltage is increased to ensure a suitable dynamic range characteristic. Thus, in the case of using a hard reset, the difference between a reset level of the photodiode and a reset level of the floating diffusion node in the photodiode reset period decreases and may cause a lag phenomenon.
To prevent this lag phenomenon, it has been proposed to employ a soft reset. However, a soft reset may result in a substantial reset level variation may occur during a read-out operation.